1. Field of the Invention
The present invention relates to a method for producing a light-emitting diode (LED), and more particularly to a liquid phase epitaxial growth method used for producing a high-intensity GaP olive color LED (i.e., an LED having a high light-emitting efficiency) for use in outdoor display apparatus or the like.
2. Description of the Related Art
In general, LEDs used in display apparatus or the like consume less electric power with increased light-emitting efficiency. Therefore, higher light-emitting efficiency has been demanded particularly in LEDs for use outdoors.
GaP olive color LEDs have a low light-emitting efficiency because GaP crystal is an indirect transition type semiconductor. However, such diodes can emit light with a satisfactory efficiency by introducing nitrogen (N) into the GaP crystal. GaP containing N is used for a conventional high-intensity GaP olive color LED. In the following description, GaP containing N will be referred to as GaP:N.
In order to allow GaP:N LEDs to have a high light-emitting efficiency, it is important to improve the crystallinity of a pn junction.
The GaP:N LEDs can be generally produced by using a liquid phase epitaxial growth method. FIG. 5A is a diagram schematically showing epitaxial growth layers of GaP:N LED; FIG. 5B is a carrier concentration profile of each layer thereof.
As shown in FIGS. 5A and 5B, an n-type GaP layer 21 is formed on an n-type GaP substrate 20 with a heat-melted Ga containing a dopant and phosphorus (P) (hereafter, referred to as melted Ga) by liquid phase epitaxial growth. At this time, the surface of the n-type GaP substrate 20 is generally melt back with the unsaturated melted Ga to obtain crystal lattice matching. After the n-type GaP layer 21 is grown to a predetermined thickness (A of FIG. 5B), an n-type GaP:N layer 22 containing N is formed on the n-type GaP layer 21. In general, N is introduced into the n-type GaP:N layer 22 by bringing NH.sub.3 gas into contact with the melted Ga.
After the n-type GaP:N layer 22 is grown to a predetermined thickness (B of FIG. 5B), a p-type GaP:N layer 23 (C of FIG. 5B) is formed thereon. Here, Zn is used as a p-type dopant. The Zn can be obtained by bringing vaporized Zn into contact with the melted Ga.
Assuming that the carrier concentration of the n-type GaP layer 21, the n-type GaP:N layer 22, and the p-type GaP:N layer 23 are respectively n.sub.1, n.sub.2, and p, the relationships: n.sub.1 &gt;n.sub.2, n.sub.2 &lt;p can be generally obtained. Alternatively, in the case where the p-type GaP:N layer 23 is divided into two layers and the respective carrier concentrations are assumed to be p.sub.1 and p.sub.2, the relationships: n.sub.1 &lt;n.sub.2, p.sub.1 &lt;p.sub.2, and n.sub.2 &gt;p can be obtained (Japanese Patent Publication No. 60-19676).
In the case where the carrier concentration satisfies the relationship: n.sub.2 &lt;p, there arises a problem in that an implantation efficiency is low although the crystallinity of a pn junction is relatively good. In the case where the carrier concentration satisfies the relationship: n.sub.2 &gt;p, there arises a problem in that a crystal defect is likely to be introduced into the crystal at the pn junction by donors although the implantation efficiency is better than the implantation efficiency in the case of n.sub.2 &lt;p.
Furthermore, in the case where the n-type GaP layer 21 is grown by melting back the surface of the n-type GaP substrate 20 with the melted Ga, an n-type dopant and other impurities are mixed in the melted Ga. As a result, the carrier concentration at the pn junction obtained by forming the p-type GaP:N layer 23 on the n-type GaP:N layer 22 cannot be sufficiently controlled, which allows a non-light-emitting level to be readily formed.
Furthermore, while the surface of the n-type GaP substrate 20 is melt back. The distribution of the partial pressure of P becomes non-uniform at the interface since the partial pressure of P is small at the interface between the n-type GaP substrate 20 and the melted Ga. As a result, the composition of crystal becomes non-uniform, and the quality of the crystal of the epitaxial growth layer is likely to decrease.